I am collecting books, making my kids a library. What are some good books to look for? Not only kids books.?

Trawl around garage sales and thrift stores and anywhere you can get books dirt cheap. Then you can afford to take some chances and build a broad library without breaking the bank. Plenty of solid reference books, science, history, philosophy, stuff you've heard of but maybe never read. Same with clic literature. Do a search on 'Penguin Clics' for some ideas but if you recognise a title when you're browsing those piles of second hand books then it's worth considering. For example a local charity shop had a lovely illustrated copy of Homer's Iliad which cost approx 50 cents, so I got it for my daughter's collection even though she's only 4 and won't be interested in it for years.